1. Field of the Invention
The present invention relates to a three-plate optical system and a projector, for example, a three-plate image projection optical system equipped with a reflective display element such as a digital micromirror device and with a color separating/combining prism, and a projector including the three-plate image projection optical system.
2. Description of Related Art
A digital micromirror device is known as a reflective display element mounted on a projector. The digital micromirror device includes an image display surface formed with a plurality of microscopic micromirrors. On the image display surface, the digital micromirror device controls tilt of each of mirror surfaces to modulate intensity of illumination light, thereby forming an image. An ON/OFF of each of pixels of the digital micromirror device is indicated, for example, by mirror surface pivoting of ±12° around a rotation axis forming an angle of 45° with respect to each of sides of the image display surface.
On a projector using a reflective display element such as a digital micromirror device and using a color separating/combining prism, an incident angle with respect to a dichroic coating inside the color separating/combining prism differs between illumination light and projection light (ON light). This leads to occurrence of a light quantity loss due to an angular characteristic difference of the dichroic coating. To reduce the light quantity loss, an optical system described in Patent Literature 1 has improved the angular characteristic of a coating, and an optical system described in Patent Literature 2 has designed a prism shape that reduces an incident angle difference between an illumination path and a projection path.